Re: Classical and Quantum Computers

Stuart Hameroff wrote
> How are dendritic inputs integrated to trigger axonal action
> potentials? The standard dogma, that dendritic membrane
> potentials summate to a trigger threshold, does not work (or
> so I have been told by people who work in this area).

> It seems likely to me that microtubule networks of the
> dendritic cytoskeleton perform this integration. Moreover (I
> would argue), quantum computations in the microtubules of the
> dendritic cytoskeleton may be required, and these processes
> are closely tied to consciousness. (Eccles, Pribram and others
> have long contended that consciousness occurs in dendrites).
> This is described in some details in the Penrose-Hameroff
> [Orchestrated Objective Reduction] model.

Why are *quantum* computations required?  What precisely is
computed and why can it not be computed conventionally?

Penrose claims that consciousness somehow has
``non-computational aspects'', but, as I have tried to explain
elsewhere in this thread, the current physicists' understanding
of a quantum computer does not lead to non-computational
processes; at least as long as it is assumed that infinite
computations are not somehow being performed.  To produce a
system which can perform infinite computations would
certainly be much harder, perhaps even infinitely harder, than
just producing an ``ordinary'' quantum computer.

I wrote
>  evolution does not tune something unless it is already
>  producing some sort of biological benefit

and
>  The ability rapidly to factor large integers would not
>  help a plant to survive in the jungle, but even if it did, no path
>  to an evolved number-factoring quantum computer could exist
>  because there are so many easier ways to factor small
>  numbers.

and
>   Mother nature works with the technology of the day
>  constantly demanding useful improvements in function; she
>  has no time for blue skies research.  Wings can evolve because
>  even a little flight -- a long jump -- is useful.  Eyes can
>  evolve because even a little sight -- shadow detection -- is
>  useful.  A little quantum computation, however, is just a very
>  expensive ordinary computation.

Hameroff replied
>  Evolution has had several billion years to develop our brains.

When, in course of that several billion years, was the jump from
classical to quantum computation?  By what steps was the jump
achieved?  What did it then become possible to compute?  What
advantage was thereby derived?

Almost as soon as I had written these questions, I discovered
that Hameroff had proposed answers to them in a paper on his
web site:

http://www.consciousness.arizona.edu/hameroff/Pen-Ham/Cambrian_Explosion/Cambrian%20Explosion.htm

`` Did Consciousness Cause the Cambrian Evolutionary
Explosion? ''
( Toward a Science of Consciousness II: The 1996
Tucson Discussions and Debates
Editors Stuart Hameroff, Alfred Kaszniak, Alwyn Scott
 MIT Press, Cambridge MA 1998, pp.421-437)

In this paper, Hameroff suggests that, ``Small worms, urchins
and comparable creatures reached critical biological complexity
for emergence of primitive consciousness at the early Cambrian
period 540 million years ago''.

He says that, ``non-computable, seemingly random conscious
choices with an element of unpredictability may have been
particularly advantageous for survival in predator-prey
dynamics''.

I would agree that an element of unpredictability is
advantageous, but it can easily be achieved conventionally in
biological systems.  Moreover, I have no idea how what sort of
advantageous choices worms might be capable of that could not
be adequately modelled on a conventional computer with a
random number generator.  Indeed, despite reading several
papers/books by Penrose/Hameroff, I remain doubtful that
``non-computability'' has anything to do with ``choice''.

I too believe that ``reduction'' is important to consciousness
(and consciousness to reduction), but I dispute the
orchestration (and the objectivity).  But even if there was
orchestration, I don't see what is gained by supposing ``quantum
computation'' to be a part of the process.


In his recent posting, Hameroff also mentions Max Tegmark's
paper, ``The importance of quantum decoherence in brain
processes'' quant-ph/9907009 and his response to it:
S. Hagan, S.R. Hameroff, and J.A. Tuszynski, `Quantum
computation in brain microtubules? Decoherence and biological
feasibility'' quant-ph/0005025.

I agree that Tegmark's analyses are fairly crude and accept
that the specific superpositions ruled out by Tegmark are not
those claimed in the Penrose-Hameroff model.   Nevertheless,
the Hagan, Hameroff, Tuszynski paper also seems to me to have
flaws.  I shall mention one issue here.  It is minor in terms of
its effect on the calculations, but I believe that it points to
broader and more important difficulties:

One of the ways in which Tegmark's analyses are crude is that
he neglects the difference between the dielectric permittivity
of the vacuum and that of the neural medium.  Hagan, Hameroff,
and Tuszynski point out that the medium's dielectric constant
may be quite high, but apparently fail to notice that this is
precisely because the medium, considered as an environment, is
sensitive to the movement of charges and therefore is itself
decohering.  To argue for decoherence, it is only necessary to
demonstrate one decohering mechanism, but coherence requires
that every such mechanism be excluded.


Matthew Donald ([EMAIL PROTECTED])
web site:
http://www.poco.phy.cam.ac.uk/~mjd1014
``a many-minds interpretation of quantum theory''

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